This invention relates to a tougher and more tear resistant polyester film. More particularly, it relates to an exterior backing member for a tape-based easy opening can closure system.
Aesthetic and environmental considerations have recently motivated the development of beverage container closure systems which can be easily opened and which do not generate non-biodegradable container portions, e.g., ring-pull tops, that can be carelessly discarded into the environment. One solution to this problem is that disclosed in U.S. Pat. No. 3,990,603 (Brochman) incorporated herein by reference. Brochman employs an exterior tape and an interior sheet situated externally and interiorly (respectively) circumjacent a preformed opening (i.e., the pour hole) in the end portion of a beverage container, the exterior tape and interior sheet adhering to each other in the area of the preformed opening. In the opening operation, removal of the exterior film pulls the interior sheet material through the preformed opening in the beverage container shearing the interior sheet along the edges of the preformed opening, thereby giving access to the contents. Brochman discloses the parameters that must be met by the exterior tape and interior film in order to suitably contain a carbonated beverage under the conditions encountered in canning and in the ultimate consumption of carbonated beverages by the consumer.
The exterior tape in a two tape container closure system comprises a backing member on which is firmly anchored an adhesive layer. Conventional polyester films, e.g., polyethylene terephthalate (PET) have been found suitable as exterior tape backing members where containment of carbonated beverages is desired. However, in some applications it was preferable to employ a coextruded backing member, e.g., a PET layer coextruded with a copolyester comprising the reaction product of ethylene glycol and 80% terephthalate - 20% isophthalate, and adhesive primers, e.g., polycarbodiimides, to prevent delamination of the adhesive from the PET backing member. In other applications, straight PET films demonstrated inadequate tear strength or toughness. Polytetramethylene terephthalate or polybutylene terephthalate (unless the context indicates otherwise, only the 1,4 isomer is intended) films were found to have advantageous tear strength and toughness. However, PTMT resins are relatively expensive and were found difficult to process due to their rapid rates of crystallization after extrusion.